Catalytic cracking, and particularly fluid catalytic cracking (FCC), is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. There is, however, an increasing need to enhance the yield of light olefins, especially C.sub.3 to C.sub.5 olefins, in the product slate from catalytic cracking processes. For example, C.sub.3 to C.sub.5 olefins are useful in making ethers and alkylate which are in high demand as octance enhancing additives for gasoline.
Conventional processes for catalytic cracking of heavy hydrocarbon feedstocks to gasoline and distillate fractions typically use a large pore molecular sieve, such as zeolite Y, as the primary cracking component. It is also well-known to add a medium pore zeolite, such as ZSM-5, to the cracking catalyst composition to increase the octane number of the gasoline fraction. As disclosed in U.S. Pat. No. 4,828,679, conventional ZSM-5 cracking additives have a crystal size in excess of 0.2 micron since smaller crystal materials have reduced hydrothermal stability and hence rapidly lose activity when exposed to the high temperature steam generated during FCC regeneration.
U.S. Pat. No. 5,472,594 discloses that the yield of C.sub.4 and C.sub.5 olefins in catalytic cracking can be enhanced by adding a phosphorus-containing medium pore zeolite, such as ZSM-5, to a conventional zeolite Y cracking catalyst such that the weight ratio of phosphorus-containing medium pore zeolite to zeolite Y is in the range 0.005 to 0.10. U.S. Pat. No. 5,472,594 is, however, silent as to the crystal size of the ZSM-5.
According to the present invention, it has now been found that the addition of a phosphorus-containing, medium pore zeolite, such as ZSM-5, having a crystal size less than 0.1 micron to a conventional large pore molecular sieve cracking catalyst increases the yield of C.sub.3 to C.sub.5 olefins in the catalytic cracking of hydrocarbon feedstocks without significant loss in the aging characteristics of the medium pore additive.